ITO (indium-tin oxide) thin films are characterized by high electrical conductivity and high visible-light transmission, and have an advantage that fine processing by lithography is easy. ITO thin films are hence used in a wide range of fields, e.g., display electrodes for flat panel displays, solar cell windows, and antistatic conductive films. In particular, with the recent progress in size increase and finer quality in flat panel displays including liquid crystal displays, there is an increasingly growing demand for ITO thin films for use as display electrodes therein.
The processes for producing ITO thin films are classified into chemical film deposition techniques, e.g., spray pyrolysis process and CVD process, and physical film deposition techniques, e.g., electron beam evaporation process and sputtering process. Among these, sputtering process is used in various fields because high performance films having large areas can be easily obtained by this film deposition technique.
In producing an ITO thin film by sputtering, the target used is either an alloy target comprising indium metal and tin metal (hereinafter abbreviated as "IT target") or a composite oxide target comprising indium oxide and tin oxide (hereinafter abbreviated as "ITO target"). Compared to the sputtering process using an IT target, the process using an ITO target yields films which change less in resistance and transmission with the lapse of time and the control of conditions for film formation can be easier. Because of these advantages, the process using an ITO target has become the mainstream in processes for producing ITO thin films.
Where an ITO target is continuously sputtered in an atmosphere of argon-oxygen gas mixture, a black matter called nodule accumulates on the target surface as the integrated sputtering time increases. This black matter, which is thought to be a suboxide of indium, tends to cause arcing during sputtering because it appears on the periphery of the erosion race track of the target. It is also known that the black matter itself can be a source of particles.
As a result of the continuous sputtering, the thin films formed have defects therein. These defects have been a cause of a reduced yield in the production of flat panel displays, e.g., liquid crystal displays. In particular, in the field of flat panel displays, where there is the progress toward finer quality, the elimination of such defects in the thin films has been a serious problem which should be especially overcome because these defects cause failure of elements.
In prior art processes for producing ITO thin films, a measure to prevent the generation of such defects in thin films has been taken which comprises periodically removing the nodules on the target surface. However, this target cleaning operation has resulted in a considerable decrease in productivity.
It is known that the formation of nodules on ITO targets becomes less with increasing a density of ITO sintered body. However, a sufficiently high sintering density has not been obtained so far, and there has been a strong desire for the development of an ITO target less liable to cause the nodule formation.